A conventional resistance welding apparatus for welding the strands composed of twisted copper wires of a pair of electric cables, for example, comprises an apparatus using an AC thyristor system (a system in which thyristors are used as electric source switches for performing welding current regulation continuously by changing the firing phase of the thyristors) as shown in FIG. 8(a). The resistance welding apparatus 1 is designed to perform resistance welding on a stack of strands 31 and 31' composed of twisted copper wires of a pair of electrically insulating coated electric cables 30 and 30' (hereinafter simply referred to as "cables") as illustrated in FIG. 9. The resistance welding apparatus 1 has a box-like apparatus body 2 which is substantially U-shaped in side view. A cable-setting jig 3 is disposed in the center of the apparatus body 2. An air cylinder 4 is attached to the upper front of the apparatus body 2. A pair of upper and lower electrodes 5A and 5B pass a welding current therebetween and are provided below the air cylinder 4, and the cable-setting jig 3 respectively. These thus serves to pass the current through a welding portion of the strands 31 and 31' and also serve to apply a predetermined amount of pressure to the welding portion.
The upper electrode 5A is connected to a piston rod 4a of the air cylinder 4 through an electrode holder 6 so as to move vertically. Further, the upper electrode 5A is also connected, through an ounce copper plate 8, to a welding transformer (electric source) 7, which serves to supply a welding current. Further, the lower electrode 5B is fixed to the center portion of the apparatus body 2 and is connected to the welding transformer 7. As shown in FIG. 8(b), the welding transformer 7 is connected to a welding timer 9, which serves to set the current value and current-conduction time of the welding current. An electromagnetic valve 4A of the air cylinder 4 is opened/closed on the basis of conduction-start and conduction-end signals obtained from the welding timer 9. As shown in FIG. 9, each of the electrodes 5A and 5B is constituted by a columnar chromium-copper matter 5a and a rectangular tungsten tip 5b.
The step of performing resistance welding of the overlapping strands 31 and 31' of the pair of cables 30 and 30', by means of the AC thyristor system resistance welding apparatus 1 as shown in FIG. 9, will be described with reference to a flow chart shown in FIG. 10. First, after the exposed strands 31 and 31' of the pair of cables 30 and 30' are put in between the pair of electrodes 5A and 5B through the cable-setting jig 3, a start input switch 9A is turned on so that the welding timer 9 starts (step S1). As a result, the electromagnetic valve 4A, that is connected to a (not-shown) compression air source, is opened and the upper electrode 5A is moved down by the air cylinder 4. After completion of initial pressure application to the strands 31 and 31' between the pair of electrodes 5A and 5B (step S2), a welding current is passed between the pair of electrodes 5A and 5B alternately upward and downward by the welding transformer 7 (step S3). The welding current is passed for the current-conduction time which is set (fixed) in advance. Resistance heating caused by the conduction of the welding current is utilized so that the strands 31 and 31' are subjected to thermo-compression bonding. After resistance welding, the current conduction is stopped (step S4). Then, cooling is performed while the pressure application state between the pair of electrodes 5A and 5B is held for a predetermined time (step S5). The operation for the steps S2 to S5 is carried out automatically under the sequence control of the welding timer 9. Then, when the pressurized state is canceled, the resistance welding is completed (step S6). Such technique is disclosed in Japanese Patent Unexamined Publication Nos. Hei-1-278973 and Hei-5-329661.
The inter-tip resistance (calculated on the basis of an inter-tip voltage value and a current value under current conduction) between the pair of electrodes 5A and 5B, which shows a correlation with the adhesive force, is used in a non-destructive inspection method for inspecting the characteristics of the weld, such as the welding strength (inter-wire adhesive force), or the like, that bond the twisted copper wires in strands 31 and 31'. As shown in FIG. 11, in the case where the twisted copper wire in strands 31 and 31' are welded, these are two noteable effects. First, the inter-tip resistance value in an initial stage of current conduction (up to about 2 cycles in the case of an AC system) is high compared with that in copper plates, or the like, because a space is generated between the strands 31 and 31'. Second, variations occur in contact resistance because the state of the alignment of the strands 31 and 31' is not uniform. Accordingly, when the inter-tip resistance is measured during the entire period of current conduction and averaged to evaluate the state of the weld, the inter-tip resistance is greatly affected by the contact resistance between the strands 31 and 31' in the initial stage of current conduction. As a result, the state of heating (increase of specific resistance) of the pair of electrodes 5A and 5B made from tungsten, or the like, cannot be determined accurately, so that the welding strength (adhesive force) cannot be predicted.
Therefore, the present invention is designed to solve the above problem. Accordingly, it is an object of the present invention to provide a method for inspecting the quality of a resistance weld, in which the adhesive force between the twisted wires of the strands as materials to be welded can be predicted on the basis of the inter-tip resistance between electrodes so that the welding quality can be evaluated accurately.